Danny,
Your question is one that is frequently asked on this Reflector. Since
no one else has responded to your posting, I'd like to offer my
understanding. You ask "is there some way to enhance the Sigma 80 (or any
other vertical antenna) performance and imitate salt water?"
Guy Olinger, K2AV, summarizes the loss mechanisms for vertical antenna
systems in this manner:
1) Loss in the "current sink" - solved with a dense radial system, elevated
radials, or a vertical dipole (an elevated radial antenna)
2) Loss from E plane penetration in the ground below and from adjacent
conductive structures, "near field" losses. Part of this loss is reduced by
a dense ground screen right at the base of the antenna. The idea is to make
the ground immediately underneath appear highly conductive. This approach
has been recommended by Uli Weiss, DJ2YA, who edited the Vertical Antenna
section of ON4UN's Low Band DXing book, and others for 1/4 verticals with
elevated radials. This loss mechanism is much less with vertical dipole
antennas. The ground current density from a vertical dipole is much reduced
compared to the typical 1/4 vertical mounted close to the ground. Antenna
location can also be an important means to reduce these losses as well.
Nearby foliage and nearby structures such as towers, buildings, fences,
power lines..........etc, can increase these "near field" losses. Placing
the antenna away from or above these conductive structures reduces the
coupling and potential loss from the vertical antenna to them.
3) Loss in the first bounce in the antenna "far field", many meters if not a
kilometer away from the antenna. This loss markedly affects the lowest
radiation angles, those most useful for DX. This is the loss that is so
markedly reduced by a seaside location. As you can see from the above
theory, a dense ground screen is not a practical solution to this loss
mechanism.
In summary, I don't think that one can imitate salt water.
One can reduce other losses and substantially improve vertical antenna
performance salt water or not. The Sigma 80 antenna is a 1/2 wave loaded
vertical dipole constructed from heavy duty aluminum tubing. It is only 36'
long and can be guyed at either 1 or 2 levels. It is less efficient than an
ideal vertical dipole because of the losses in the loading mechanism.
However, one potential performance advantage of the Sigma 80 style of
antenna is that the antenna can be located in an optimal location, a place
where a wire vertical or a ground screen or elevated radials is not
possible. For instance, I was able to place my Sigma 80 antenna in a clear
space in the garden away from everything where there are no trees to support
a wire, and no place on the ground for radials of any sort. I can imagine
that the Sigma 80 can also be located on the top of a house or other
building to remove the antenna from coupling to nearby conductive
structures. Other operators have mounted their Sigma 80 as a horizontal
dipole on the top of a tower.
Hope that these comments are of help to you.
John Petrich, W7FU
----- Original Message -----
From: "Danny" <ON7NQ@pandora.be>
To: <towertalk@contesting.com>
Sent: Monday, March 25, 2002 7:30 AM
Subject: Re: [Towertalk] Force 12 Sigma verticals
> John , tnx for posting ur findings on the reflector.
> They were vy useful.
> Im abt to re-join the lowband crowd and currently testdriving
> an inverted L ( with 2 elevated radials ) and a 1/4 sloper connected
> some 30ft up the tower ( i know thats way to low but .... )
> The Sigma 80 is another option , however something on the Sigma 80
> page intrigued me.
> It states : The only thing to enhance the Sigma 80 is to place it
> adjacent to salt water.
> Now , what happens if there is no salt water around ?
> Ground is avg to good around here , but not salty :)
> Is there a way to "imitate" this by using a ground screen ,
> say by using sheets of chicken wire ?
> Anyone an idea about this ?
> tnx
>
> 73 Danny - ON7NQ -
>
> ON7NQ@pandora.be
>
> http://users.pandora.be/ON7NQ
>
>
>
> ----- Original Message -----
> From: "John Petrich" <petrich@u.washington.edu>
> To: "Finger, Bob" <Bob.Finger@DynCorp.com>; <towertalk@contesting.com>
> Sent: Monday, March 18, 2002 8:42 PM
> Subject: Re: [Towertalk] Force 12 Sigma verticals
>
>
> > Bob,
> >
> > I have not actually constructed my array just yet. Have been using
a
> > single Sigma 80 on 80 CW for this past season and have compared it's
> > performance to known antennas at this QTH.
> >
> > 1) The "standard" Sigma 80 is constructed for the SSB portion of the
> > band. Mine wouldn't resonate below about 3.650 MHz. Had to order some
1"
> > tubing extensions to get resonance to 3.510 MHz. Not sure what
> frequencies
> > you are interested in, but bear this in mind if and when you order.
> >
> > 2) The bottom line is that the Sigma 80 seems to perform equally to
my
> > full sized 1/4 wave GP, 20' elevated feed and 6 elevated radials. The
> band
> > width is, of course, much narrower. My 150' dipole is better for higher
> > angle signals and less effective for LP than the vertical or the Sigma
80.
> >
> > 3) I rationalize the high cost of a Sigma 80 based on the vastly
lower
> > "hassle" factor compared to the alternatives (at this QTH) and the
reduced
> > "clap trap" factor in the garden. I am putting mine up in September and
> > taking it down in March.
> >
> > 4) My plans are to construct a fixed 2 element array for next year.
> The
> > design options are many. I'm a firm believer in Roy's, W7EL, approach
to
> > driven arrays reflected in the ARRL Antenna Handbook.
> >
> > Sorry that I cannot give you any first hand experience with the
Sigma
> 80
> > in an array. Next year. Maybe others are working on some and will
> respond.
> >
> > Regards,
> >
> > John Petrich, W7FU
> >
> >
> > ----- Original Message -----
> > From: "Finger, Bob" <Bob.Finger@DynCorp.com>
> > To: <towertalk@contesting.com>
> > Sent: Monday, March 18, 2002 11:14 AM
> > Subject: [Towertalk] Force 12 Sigma verticals
> >
> >
> > > Has anyone built a triangle or 4square array using the Force 12 Sigma
80
> > or
> > > 40 verticals? If so, what are your reactions both good and bad.
Seems
> to
> > > me like it might well be an effective solution (but expensive) to
> limited
> > > space where a full sized radial screen is not feasible. Thanks for
the
> > > comments. W9GE
> > >
> > >
> > > --- StripMime Report -- processed MIME parts ---
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> > > text/html
> > > ---
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> >
>
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